Currently available techniques for characterizing water vapor in hydrocarbon gas mixtures suffer from various drawbacks. For example, maintenance and calibration issues in the field may make their use cumbersome and costly. In addition, these techniques may be difficult to calibrate, may drift over time, and may generally fail to provide rapid response and recovery times.
One conventional technique measures the dew point of the water vapor in a gas mixture by flowing the gas mixture over a chilled mirror. Moisture in the sampled gas mixture condenses on the mirror when the mirror's temperature is at or below the dew point of the gas mixture. To estimate the water vapor concentration, the temperature of the mirror is scanned through an appropriate range from warmer to cooler, and the temperature is measured when condensation begins on the mirror surface. The dew point is a function of the relative humidity of the gas mixture, which is readily converted to a partial pressure or concentration of water vapor in the gas mixture. Detection of condensation on the mirror may be accomplished visually or by optical means. For example, a light source may be reflected off the mirror into a detector and condensation detected by changes in light reflected from the mirror. The observation may also be done by eye. However, the exact point at which condensation begins is often not readily distinguishable in this manner. Also, because the mirror temperature passes dynamically through the dew point, the error in the measurement tends to be substantial. Other, lower vapor pressure components of the gas mixture, such as higher molecular weight hydrocarbons, alcohols, and glycols, may also condense on the mirror as it cools. Automated on-line systems may be unable to distinguish between gas mixture components that condense on the mirror surface, and manual systems generally require highly skilled operators.
Another conventional technique uses two closely spaced, parallel windings coated with a thin film of phosphorous pentoxide (P2O5). An electrical potential applied to the windings electrolyzes water molecules adsorbed by the coatings to hydrogen and oxygen. The current consumed by the electrolysis reaction is proportional to the mass of water vapor entering the sensor. The flow rate and pressure of the incoming sample must be controlled precisely to maintain a standard sample mass flow rate into the sensor. However, contamination from oils, liquids or glycols on the windings causes drift in the readings and may damage the sensor. The sensor reacts slowly to sudden changes in moisture, as the absorption reaction on the surfaces of the windings takes some time to equilibrate. Large amounts of water in a gas mixture (commonly known as “slugs”) may wet the surface which requires tens of minutes or hours to “dry-down” before accurate measurements are again possible. As such, effective sample conditioning and removal of liquids is essential when using this sensor.
Still another conventional technique utilizes piezoelectric adsorption. Such an instrument compares changes in the frequency of hygroscopically coated quartz oscillators. As the mass of the crystal changes due to adsorption of water vapor on the hygroscopic coating, the resonant frequency of the quartz crystal changes. The sensor is a relative measurement that requires an integrated calibration system with desiccant dryers, permeation tubes and sample line switching. These instruments may also suffer from interference by glycol, methanol, and other polar molecules as well as from damage from hydrogen sulfide. However, the required calibration system is not as precise and adds to the cost and mechanical complexity of the system. Labor for frequent replacement of desiccant dryers, permeation components, and the sensor heads greatly increase operational costs. Additionally, as with the electrolysis-based system described above, slugs of water may render the system nonfunctional for long periods of time as the sensor head “dries-down.”
Aluminum and silicon oxide sensors have also been used. These sensors include an inert substrate material and two dielectric layers, one of which is sensitive to humidity. Water molecules in the gas mixture pass thru pores on an exposed surface of the sensor and cause a change to a physical property of the layer beneath it. In an aluminum oxide sensor, two metal layers form the electrodes of a capacitor. The dielectric constant of the sensor changes as water molecules adsorb to its surface. The sensor impedance is correlated to the water concentration. A silicon oxide sensor is an optical device whose refractive index changes as water is absorbed into the sensitive layer. When light is reflected through the substrate, a wavelength shift can be detected on the output which can be precisely correlated to the moisture concentration.
With aluminum and silicon oxide sensors, water molecules take time to enter and exit the pores so some wet-up and dry down delays will be observed, especially after a slug. Contaminants and corrosives may damage and clog the pores causing a “drift” in the calibration. As with piezoelectric and electrolytic sensors, these sensors are susceptible to interference from glycol, methanol, and other polar organic compounds. The calibration may drift as the sensor's surface becomes inactive due to damage or blockage, so the calibration is most reliable at the beginning of the sensor's life.